This application is a Continuation application of U.S. application Ser. No. 10/909,409 filed on Aug. 3, 2004, now abandoned which claims priority under 35 U.S.C. §119(a) to Patent Application No. 2003-286030 filed in Japan on Aug. 4, 2003, and 2004-174230 filed in Japan on Jun. 11, 2004, all of which are hereby expressly incorporated by reference into the present application.
This application claims priority under 35 USC 119(a) to Patent Application No. 2004-174230 filed in Japan on 11 Jun. 2004, the content of which is incorporated herein by reference in its entirety.
The present invention relates to such image capturing devices as movie-type equipment, CCD cameras, CMOS imagers, and silver halide cameras employing LED device(s) as illumination source(s); and in particular, relates to image capturing devices permitting increased effective luminance of illumination source(s) employing LED device(s).
Use of R, G, and B light emitting diodes (LEDs) as illumination light source (flash apparatus) for cameras has been proposed conventionally (see, e.g., Japanese Patent Application Publication Kokai No. 2002-116481). As compared with xenon discharge tubes and the like, use of R, G, and B LEDs is advantageous because adjustment of light source color temperature is facilitated, less time is required for power supply charging and discharging, and so forth.
Furthermore, known in connection with cinematic light sources or stroboscopic light sources for cameras for capturing images of moving subjects is a usage wherein light source(s) is/are sequentially lit (see, e.g., Japanese Patent Application Publication Kokai No. H5-328210 (1993) or Japanese Patent Application Publication Kokai No. S63-274934 (1988)). Here, light of identical color is sequentially emitted synchronously with respect to shutter timing.
However, when attempting to use LED(s) as flash apparatus(es) as in the aforementioned conventional art, there has been the problem that luminance has been insufficient. This fact will be described with reference to FIG. 8.
FIG. 8 is a graph (Id-Po characteristics) showing the relationship between LED drive current Id and luminance (optical output Po) in the conventional art.
As shown in FIG. 8, LED luminance (optical output Po) is roughly proportional to electric current (drive current Id) when the amount of current flowing through the LED is small. Here, the constant of proportionality is referred to as the luminous efficiency η(=ΔPo/ΔId). When drive current Id is large, η decreases due to generation of heat by the LED element, and optical output Po saturates at maximum value Po1 as indicated by the curve shown in FIG. 8. That is, no matter how large LED drive current is made, LED luminance does not exceed some fixed limit; and it is consequently impossible to obtain luminance as necessary for flash use.
Specific numerical values for generation of heat by chip-type LED elements follow. Results of measuring respective R, G, and B LED elements indicated, for example, that whereas junction temperature Tj for respective LED elements was between 34° and 44° C. and power consumption Pd was between 40 mW and 78 mW when Id=20 mA, junction temperature Tj was between 49° and 71° C. and power consumption Pd was between 110 mW and 230 mW when Id=50 mA. Because there is almost no change in the voltage applied at the respective LEDs, power consumption Pd should be roughly proportional to drive current Id absent any effect due to generation of heat. That is, if drive current Id increases by a factor of 2.5 in going from 20 mA to 50 mA, then power consumption Pd should also increase by a factor of 2.5, going from between 40 mW and 78 mW to between 100 mW and 195 mW. And yet, power consumption Pd actually increases by more than a factor of 2.5.
When used as a camera flash, because drive current Id will reach far greater value(s), at on the order of 200 mA for each LED element of each respective color, the effect of the heat generated thereby will be even more serious. Moreover, there has also been the problem that over the course of usage the resin that encapsulates the LED element(s) can crack due to heat (cracking) and/or separation thereof from package(s) may occur.
The present invention was conceived in light of such problems in the conventional art, and provides, in its various embodiments, an image capturing device permitting increased effective luminance of camera flash(es) employing LED(s). It also provides an image capturing device that will simultaneously permit increased flash effective luminance as well as increased life of LED(s) employed and so forth.